Turbulent flow downstream of a bileaflet mechanical heart valve is investigated using digital particle image velocimetry. Evolution of flow structures during the systole and diastole phases of a typical cardiac cycle is characterized by obtaining global flow velocity measurements in multiple cross-sections of the flow field. Instantaneous and time-averaged patterns of flow velocity, vorticity, and streamline topology are used to illustrate the interaction between the unsteady vortices that results in elevation of shear stress levels. This image-based approach can potentially lead to development of methods of control of platelet activation and provides insight into the underlying flow physics.